1. Field of the invention
The present invention relates, in general, to optical interference films. In particular, the invention relates to an interference film, formed on the outer or inner surface of a lamp, e.g., a halogen lamp, which selectively reflects light from the optical spectrum in a prescribed wavelength range.
2. Description of the related art
A halogen lamp which radiates light having a small amount of infrared rays is well known. In such a halogen lamp, a filament is disposed at the center of a glass bulb, and an optical interference film is formed on the outer surface of the bulb. The optical interference film transmits visible rays and reflects infrared rays Thus, infrared rays in the light radiated from the filament are reflected toward the filament by the optical interference film and heat the filament. As a result, a decrease in infrared rays in the radiated light and an improvement of the luminous efficiency are achieved by the interference film on the conventional halogen lamp.
The Japanese laid-open patent publication 62-105357 discloses one example of such an optical interference film, which includes high refractive index layers and low refractive index layers alternately stacked one on the other, these being a total of nine to twelve layers, or more. Each high refractive index layer includes at least one metal oxide selected from titanium oxide (TiO.sub.2), tantalum oxide (Ta.sub.2 O.sub.5) and zirconium oxide (ZrO.sub.2) as a main component, and at least one additive selected from phosphorus (P), boron (B), arsenic (As), antimony (Sb), tin (Sn), zinc (Zn), lead (Pb), potassium (K), nickel (Ni) and cobalt (Co). Each low refractive index layer includes silica (SiO.sub.2) as a main component, and at least one additive selected from phosphorus (P) and boron (B).
In the above-described conventional optical interference film, each inter-layer connection between high and low refractive index layers is strengthened by the additives. Distortion in the optical interference film caused by the difference in the heat expansion coefficient between the high and low refractive index layers is also reduced by the additives. Thus, cracking or peeling of the optical interference film can be avoided.
However, the above-described additives adversely affect the heat-resisting ability of the optical interference film, so that luminous flux from such a lamp decreases significantly with time when the lamp is in use.